A noticeable feature of SBMS\(_4\)  is that it arises as a nonlinear superalgebra, with the natural definition of the supersymmetry transformations outlined below. Nonlinearities also appear in the discussion of the asymptotic symmetries of AdS supergravity in three dimensions, where the nonlinear superalgebras of [1]}, [2]}, [3]} emerge at infinity [4]}, but we have not analyzed in the SBMS\(_4\)  case whether the nonlinearities were intrinsic or could be redefined away.
